WO2012133472A1

WO2012133472A1 - Optical transmitter and wavelength multiplexing transmission device and optical transmission method

Info

Publication number: WO2012133472A1
Application number: PCT/JP2012/058041
Authority: WO
Inventors: 裕太五江渕
Original assignee: 日本電気株式会社
Priority date: 2011-03-25
Filing date: 2012-03-21
Publication date: 2012-10-04
Also published as: JPWO2012133472A1; US20140010530A1

Abstract

An optical transmitter comprises a modulator, an output light monitoring unit and a control unit. The modulator comprises: a branch unit that branches light input to the modulator into first branched light and second branched light; a first modulation unit that performs phase modulation of the first branched light; a second modulation unit that performs phase modulation of the second branched light; a rotor that rotates the plane of polarisation of either first modulated light output from the first modulation unit or the second modulated light output from the second modulation unit; and a polarisation combination unit that combines the first modulated light and the second modulated light. The output light monitoring unit monitors the optical intensity of the combined light that is output from the polarisation combination unit. The control unit controls the first modulation unit and/or second modulation unit in accordance with the monitoring results of the output light monitoring unit. This control includes optical intensity control to make the optical intensity of the first modulated light and/or the second modulated light smaller than the maximum value of the optical intensity on the modulation curve.

Description

Optical transmitter, wavelength division multiplexing transmission apparatus, and optical transmission method

The present invention relates to an optical transmitter, a wavelength division multiplexing transmission apparatus, and an optical transmission method.

As a method for controlling the fluctuation of the optical output intensity of the optical transmitter, there is a control method for monitoring the optical output intensity in the optical transmitter and feeding back the monitoring result to the optical output intensity of the light source. However, an LD (Laser Diode) generally used as a light source has a property that the wavelength of output light also changes when the light output intensity changes. For this reason, it is not preferable to vary the light output intensity of the light source in an optical transmitter, for example, a coherent transmitter, which has strict requirements for wavelength accuracy of output light.
On the other hand, in the case of performing optical communication using a high-density wavelength division multiplexing (DWDM) method, if the wavelength control of the light source is prioritized, the light output intensity deviates from a desired value.
As a method for solving such a problem, as shown in FIG. 1, there is a method of attaching a VOA (Variable Optical Attenuator) outside the optical transmitter. Thereby, it is possible to control the light output fluctuation of the optical transmitter without changing the light output intensity of the light source. However, in the case of using this method, when performing optical communication by wavelength division multiplexing (WDM), the same number of VOAs as the number of transmission wavelengths is required. Therefore, enormous cost is required.
Other techniques relating to control of fluctuations in the optical output intensity of the optical transmitter are described in, for example, Patent Documents 1 to 3.
The optical transmitter described in Patent Document 1 performs synchronous detection using a low-frequency pilot signal. Then, the bias voltage applied to the bias electrode is controlled according to the result of the synchronous detection.
The optical transmitter described in Patent Document 2 monitors fluctuations in drive amplitude and controls drive amplitude according to the result.
The external modulator described in Patent Document 3 has an automatic bias control circuit (ABC circuit: Automatic Bias Control Circuit). The automatic bias control circuit is known as a circuit that suppresses fluctuations in optical output due to drift of the operating point of the modulation curve.

JP 2008-197639 A JP 2008-092172 A JP-A-3-251815

In the optical transmitters and modulators described in Patent Documents 1 to 3, the drive signal amplitude or the bias voltage is adjusted so that the optical output intensity is maximized. For example, in a modulator that performs modulation using a QPSK (Quadrature Phase Shift Keying) modulation method, the operating point of the bias voltage is adjusted to a NULL point in the modulation curve. The drive signal amplitude is adjusted to 2Vπ. Here, Vπ is the magnitude of a voltage that can change the phase of light by π in the modulation curve.
However, when multiplexing light of multiple wavelengths that are different from each other, simply adjusting the light output intensity to the maximum will result in differences in the light intensity between wavelengths due to differences in the amount of light propagation loss that occurs for each optical transmitter. Will occur. In order to eliminate this difference, it is necessary to adjust the light intensity by adding a VOA to the outside of each optical transmitter. That is, in order to adjust the light intensity of the output light to a desired value lower than the maximum value, it is necessary to add a VOA. Therefore, enormous cost is required.
In view of such problems, the present invention has an object to provide an optical transmitter capable of adjusting the light intensity of output light to a desired value in the optical transmitter.

The optical transmitter according to the present invention includes a modulator, an output light monitoring unit, and a control unit, and the modulator branches light input to the modulator into first branched light and second branched light. A first modulating unit that performs phase modulation of the first branched light, a second modulating unit that performs phase modulation of the second branched light, a first modulated light output from the first modulating unit, A second modulation light output from the two modulation units, a rotator that rotates one of the polarization planes, and a polarization combining unit that combines the first modulation light and the second modulation light, The output light monitoring unit monitors the light intensity of the combined light output from the polarization beam combining unit, and the control unit is based on the monitoring result by the output light monitoring unit, and is at least one of the first modulation unit and the second modulation unit. In the control, the light intensity of at least one of the first modulated light and the second modulated light is made smaller than the maximum value of the light intensity in the modulation curve. That includes a light intensity control.
A wavelength division multiplexing transmission apparatus according to the present invention includes a plurality of optical transmitters and a wavelength multiplexing unit that multiplexes wavelengths output from the plurality of optical transmitters, and each of the plurality of optical transmitters includes the optical transmitter according to the present invention. It is a transmitter.
The optical transmission method of the present invention includes a branching step for branching light into a first branched light and a second branched light, a first modulation step for performing phase modulation of the first branched light, and a phase modulation of the second branched light. A rotation step of rotating any one of the polarization planes of the second modulation step, the first modulated light modulated by the first modulation step, and the second modulated light modulated by the second modulation step; Based on the polarization combining step of combining the first modulated light and the second modulated light, the monitoring step of monitoring the light intensity of the combined light combined by the polarization combining step, and the monitoring result of the monitoring step, And a control step for controlling at least one of the modulator for performing the modulation step and the modulator for performing the second modulation step. In the control step, the light intensity of at least one of the first modulated light and the second modulated light is set. And a light intensity control step for making the light intensity smaller than the maximum value in the modulation curve. It is.
The program of the present invention includes a first modulated light generated by phase-modulating the first branched light, and a second modulated light generated by phase-modulating the second branched light, the first modulated light being different in polarization plane. A monitoring step for monitoring the light intensity of the combined light, and a control step for controlling at least one of the phase modulation of the first branch light and the phase modulation of the second branch light based on the monitoring result by the monitoring step; The control step includes a light intensity control step in which the light intensity of at least one of the first modulated light and the second modulated light is made smaller than the maximum value of the light intensity in the modulation curve.

With the optical transmitter and the control method thereof according to the present invention, it is possible to adjust the light intensity of the output light to a desired value within the optical transmitter.

FIG. 1 shows a configuration of an optical transmitter related to the present invention. FIG. 2 shows an example of the configuration of the optical transmitter according to the first embodiment of the present invention. FIG. 3 shows an example of the operation of the optical transmitter according to the first embodiment of the present invention. FIG. 4 shows an example of the configuration of an optical transmitter according to the second embodiment of the present invention. FIG. 5 shows the relationship between the modulation curves in the I arm and the Q arm, the pilot signal waveform, and the amplitude of the drive signal. FIG. 6 shows an example of the operation of the optical transmitter according to the second embodiment of the present invention. FIG. 7 shows another example of the configuration of the optical transmitter according to the second embodiment of the present invention. 8A and 8B show constellation maps of the I component and the Q component, respectively. FIG. 9 shows experimental data regarding the relationship between the operating point of the bias voltage and the amplitude of the pilot signal. FIG. 10A to FIG. 10E show experimental data relating to the waveform of the pilot signal and the output waveform when the operating point of the bias voltage is varied. FIG. 11 shows an example of the configuration of an optical transmitter according to the third embodiment of the present invention. FIG. 12 shows the relationship between the modulation curves in the I arm and Q arm, the pilot signal waveform, and the amplitude of the drive signal when the amplitude of the drive signal is varied. FIG. 13 shows an example of the operation of the optical transmitter in the fourth embodiment of the present invention. FIG. 14 shows the light intensities of a plurality of channels having different wavelengths in WDM communication. FIG. 15 shows an example of the configuration of a wavelength division multiplexing transmission apparatus according to the fifth embodiment of the present invention. FIG. 16 shows another example of the configuration of the wavelength division multiplexing apparatus according to the fifth embodiment of the present invention.

Embodiments of the present invention will be described with reference to the drawings. However, such a form does not limit the technical scope of the present invention.
[First Embodiment]
The optical transmitter according to the first embodiment of the present invention will be described with reference to FIG. FIG. 2 shows a configuration of the optical transmitter 10 in the present embodiment.
The optical transmitter 10 includes a modulator 11, an output light monitoring unit 12, and a control unit 13. The modulator 11 includes a branching unit 14, a first modulating unit 15, a second modulating unit 16, a rotator 17, and a polarization beam combining unit 18. The branching unit 14 branches the light input to the modulator 11 into a first branched light and a second branched light. The first modulation unit 15 performs phase modulation of the first branched light. The second modulation unit 16 performs phase modulation of the second branched light. The rotator 17 rotates one of the polarization planes of the first modulated light output from the first modulation unit 15 and the second modulated light output from the second modulation unit 16. The polarization beam combiner 18 combines the first modulated light and the second modulated light.
The output light monitoring unit 12 monitors the light intensity of the combined light output from the polarization beam combining unit 18.
The control unit 13 controls at least one of the first modulation unit 15 and the second modulation unit 16 based on the monitoring result by the output light monitoring unit 12. The control performed by the control unit 13 includes control for making the light intensity of at least one of the first modulated light and the second modulated light smaller than the maximum value of the light intensity in the modulation curve.
Next, the operation of the optical transmitter 10 will be described with reference to FIG.
First, the light input to the modulator 11 of the optical transmitter 10 is branched into the first branched light and the second branched light by the branching unit 14 (step 1). Then, the first branched light is phase-modulated by the first modulation unit 15 and becomes the first modulated light. Further, the second branched light is phase-modulated by the second modulation unit 16 to become second modulated light (step 2). The first modulated light and the second modulated light are combined by the polarization beam combiner 18 (step 3). The combined light output from the polarization beam combiner 18 is output from the optical transmitter 10.
Here, the output light monitoring unit 12 monitors the light intensity of the combined light output from the polarization beam combining unit 18 (step 4). The output light monitoring unit 12, for example, as in a second embodiment described later, is based on the output from the photoelectric conversion element into which the output light from the polarization beam combining unit 18 is branched and input, and the combined light It is good also as monitoring intensity. Alternatively, as in a third embodiment to be described later, based on the light intensity of the first modulated light, the light intensity of the second modulated light, and information on the light loss recorded in the recording unit, the light of the synthesized light It is good also as monitoring intensity. Thus, the output light monitoring unit 12 only needs to be able to monitor the light intensity of the combined light output from the polarization beam combining unit 18 by any method, and the specific configuration is not limited.
The control unit 13 controls at least one of the first modulation unit 15 and the second modulation unit 16 based on the monitoring result by the output light monitoring unit 12 (step 5).
Here, the control by the control unit 13 includes control for making the light intensity of at least one of the first modulated light and the second modulated light smaller than the maximum value of the light intensity in the modulation curve. That is, the control unit 13 performs control to darely reduce the light intensity below the maximum value in the modulation curve depending on the monitoring result by the output light monitoring unit 12. For example, when it is found from the monitoring result by the output light monitoring unit 12 that the light intensity of the combined light is higher than a desired value, the control unit 13 controls the first modulation unit 15 to control the first modulation. It is also possible to reduce the light intensity. Alternatively, the control unit 13 may control the second modulation unit 16 to reduce the light intensity of the second modulated light. Alternatively, the control unit 13 may control both the first modulation unit 15 and the second modulation unit 16. Thus, the control unit 13 reduces the light intensity of the combined light by reducing the light intensity of at least one of the first modulated light and the second modulated light.
In this way, the optical transmitter 10 can attenuate the light intensity of the combined light output from the polarization beam combiner 18 inside the optical transmitter 10 according to the monitoring result by the output light monitoring unit 12. it can.
Then, by repeating the steps from Step 1 to Step 5, the light intensity of the output light from the optical transmitter 10 can be converged to a desired value.
As described above, the optical transmitter 10 in the present embodiment can adjust the light intensity of the output light to a desired value by controlling at least one of the first modulation unit 15 and the second modulation unit 16. it can. Therefore, it is not necessary to add VOA outside the optical transmitter 10 in order to set the optical output intensity of the optical transmitter 10 to a desired value. Similarly, it is not necessary to provide a VOA inside the optical transmitter 10. Therefore, according to the present embodiment, it is possible to reduce the component cost as compared with the case where the VOA is added inside or outside the optical transmitter.
In Patent Documents 1 to 3, only control based on the light intensity of one polarized wave is described. For this reason, no consideration is given to optical loss or the like that occurs when two polarizations are combined. On the other hand, in the present embodiment, the light intensity of the combined light obtained by combining the two polarized waves is monitored. Therefore, even when polarization combining is performed in the optical transmitter, the optical output intensity can be accurately controlled.
[Second Embodiment]
An optical transmitter according to the second embodiment of the present invention will be described with reference to FIG. FIG. 4 shows a configuration of the optical transmitter 20 in the present embodiment.
The optical transmitter 20 in the present embodiment includes a light source 21, a modulator 22, an output light monitoring unit 23, a control unit 24, a bias circuit 25, a drive circuit 26, and an external photoelectric element 27. The modulator 22 includes a branching unit 28, a first modulating unit 29, a second modulating unit 30, a rotor 31, a polarization beam combining unit 32, a first internal photoelectric element 33, and a second internal photoelectric element 34. And having.
The optical transmitter 20 in the present embodiment performs optical transmission using a DP-QPSK (Dual Polarization-Differential Quadrature Phase Shift Keyin) method. The DP-QPSK method is a method in which 2-bit data can be assigned to each of four modulated lights with respect to two orthogonal polarizations.
Each of the first modulation unit 29 and the second modulation unit 30 includes two Mach-Zehnder interferometers that constitute an I arm and a Q arm, and performs quaternary phase modulation (QPSK modulation). Further, a phase shift unit 35 is provided on the output side of each arm. ₁ ~ 35 ₄ Is placed. Phase shift unit 35 ₁ ~ 35 ₄ Respectively gives a relative phase difference to the light propagating through the I arm or the Q arm. In QPSK modulation, [θ], [θ + π / 2], [θ + π], [θ] for each symbol [00], [10], [11], [01] composed of 2-bit data, respectively. [Θ + 3π / 2] is assigned. Here, θ is an arbitrary phase.
The external photoelectric element 27 is disposed on the output side of the modulator 22 and receives a part of the combined light output from the polarization beam combiner 32. The polarization beam combiner 32 is also called a polarization beam combiner (PBC), and is an optical coupler that combines a plurality of lights having polarization planes with different angles.
The first internal photoelectric element 33 is disposed on the output side of the first modulation unit 29 and receives a part of the first modulated light output from the first modulation unit 29. The second photoelectric element 34 is disposed on the output side of the second modulation unit 30 and receives a part of the second modulated light output from the second modulation unit 30. The external photoelectric element 27, the first internal photoelectric element 33, and the second internal photoelectric element 34 are photoelectric conversion elements such as PD (Photo Diode), for example.
The output light monitoring unit 23 monitors the light intensity of the combined light based on the output from the external photoelectric element 27. Further, based on the output from the first internal photoelectric element 33, the light intensity of the first modulated light is monitored. Further, the light intensity of the second modulated light is monitored based on the output from the second internal photoelectric element 34.
The control unit 24 controls the light source 21, the bias circuit 25, and the drive circuit 26. Note that the control unit 24 in the present embodiment controls the bias circuit 25 based on the monitoring result by the output light monitoring unit 23, thereby applying a bias to be applied to each arm of the first modulation unit 29 and the second modulation unit 30. Control the voltage.
The bias circuit 25 applies a bias voltage to the first modulation unit 29 and the second modulation unit 30. Specifically, the bias circuit 25 includes an I arm and a Q arm included in the first modulation unit 29 and the second modulation unit 30, and a phase shift unit 35. ₁ ~ 35 ₄ For each, a bias voltage is applied.
The drive circuit 26 inputs drive signals to the first modulation unit 29 and the second modulation unit 30. Specifically, the drive circuit 26 inputs drive signals to the I arm and the Q arm included in the first modulation unit 29 and the second modulation unit 30, respectively.
The relationship among the modulation curves in the I and Q arms of the first modulation unit 29 and the second modulation unit 30, the pilot signal waveform, and the amplitude of the drive signal will be described with reference to FIG.
First, a graph of a modulation curve in each arm of the modulator 22 will be described. The vertical axis in this graph indicates the light output intensity of the output light from each arm. The horizontal axis indicates the magnitude of the bias voltage applied to each arm of the modulator 22. In the QPSK modulation method, the input signal is generally modulated with a drive signal having an amplitude of 2Vπ in this modulation curve. Here, Vπ is a voltage that can change the phase of light by π in the modulation curve. FIG. 5 also shows the relationship between the amplitude of the drive signal and the modulation curve. From this graph, it can be seen that the light intensity of the output light can be controlled by controlling the operating point of the bias voltage input to each arm of the modulator 22.
In the present embodiment, a pilot signal having a constant frequency and amplitude is superimposed on the bias voltage applied to each arm. The output light monitoring unit 23 detects the demodulated pilot signal from the output of the first internal photoelectric element 33. Specifically, the output light monitoring unit 23 removes a DC (Direct Current) component from the electrical signal output from the first internal photoelectric element 33 and extracts only an AC (Alternate Current) component. As a result, the output light monitoring unit 23 detects the demodulated pilot signal. Similarly, the output light monitoring unit 23 detects the demodulated pilot signal from the output from the second internal photoelectric element 34. FIG. 5 also shows the waveform of the pilot signal superimposed on the bias voltage and the waveform of the demodulated pilot signal.
FIG. 5 shows that the amplitude of the demodulated pilot signal is minimized when the bias voltage and the drive signal amplitude are controlled so that the light output intensity is maximized (PEAK point). That is, when the operating point of the bias voltage is adjusted to the NULL point in the modulation curve and the amplitude of the drive signal is 2Vπ, the amplitude of the pilot signal is minimized and the light output intensity is maximized. In other words, the light output intensity is maximized by controlling the bias voltage and the drive signal amplitude so that the amplitude of the pilot signal is minimized. It can also be seen that when the bias voltage is controlled so that the optical output intensity is intermediate (QUADRATURE point) between the maximum (PEAK point) and minimum (NULL point), the amplitude of the demodulated pilot signal is maximum. . For example, if the amplitude of the drive signal is not changed from 2Vπ and the operating point of the bias voltage is shifted by Vπ / 2 from the NULL point, the amplitude of the demodulated pilot signal becomes maximum. Note that the phase of the demodulated pilot signal differs depending on whether the operating point of the bias voltage is shifted to the left from the NULL point or to the right from the NULL point. Here, the fact that the operating point of the bias voltage is shifted leftward from the NULL point indicates that the bias voltage is shifted in the direction of decreasing. Here, the fact that the operating point of the bias voltage shifts to the right from the NULL point indicates that the bias voltage shifts in the direction in which the bias voltage increases. That is, by detecting the amplitude and phase of the demodulated pilot signal, it is possible to determine how much the operating point of the bias voltage is shifted from the NULL point in which direction.
Next, the operation of the optical transmitter 20 will be described.
First, the light output from the light source 21 of the optical transmitter 20 is branched into the first branched light and the second branched light by the branching unit 28.
The first branched light is input to the first modulation unit 29. The first branched light input to the first modulation unit 29 propagates through the I arm and the Q arm included in the first modulation unit 29 and is phase-modulated in each arm. Here, the bias voltage output from the bias circuit 25 and the drive signal output from the drive circuit 26 are input to the I arm and the Q arm of the first modulation unit 29, respectively.
The first branched light is phase-modulated by the first modulation unit 29 to become first modulated light, and is output from the first modulation unit 29. At this time, a part of the first modulated light is input to the first internal photoelectric element 33. The first internal photoelectric element 33 converts the input optical signal of the first modulated light into an electrical signal.
The output light monitoring unit 23 monitors the light intensity of the first modulated light based on the output of the first internal photoelectric element 33 and inputs the monitoring result to the control unit 24. Specifically, the output light monitoring unit 23 detects the amplitude and phase of the demodulated pilot signal by extracting the AC component of the electrical signal output from the first internal photoelectric element 33. Here, the output light monitoring unit 23 monitors the light intensity of the first modulated light by utilizing the correlation shown in FIG. 5 between the amplitude of the demodulated pilot signal and the light output intensity. Therefore, the output light monitoring unit 23 records information on the relationship between the modulation curve and the pilot signal of each arm included in the first modulation unit 29 and the second modulation unit 30. Specifically, the output light monitoring unit 23 records information on the relationship among the amplitude of the pilot signal, the phase of the pilot signal, the light intensity, and the bias voltage as shown in FIG. Accordingly, the output light monitoring unit 23 can monitor the light intensity of the first modulated light based on the amplitude information of the demodulated pilot signal.
The control unit 24 controls the bias voltage applied to the first modulation unit 29 based on the monitoring result input from the output light monitoring unit 23. For example, when it is determined from the monitoring result of the output light monitoring unit 23 that the light intensity of the first modulated light is larger than a desired value, the light intensity attenuates the bias voltage applied to the first modulation unit 29. To control. At this time, the control unit 24 refers to the phase of the demodulated pilot signal to determine whether the bias voltage should be controlled to be increased or decreased.
On the other hand, the second branched light output from the branching unit 28 is input to the second modulation unit 30. The second branched light input to the second modulation unit 30 propagates to the I arm and the Q arm included in the second modulation unit 30 and is phase-modulated in each arm. Here, the bias voltage output from the bias circuit 25 and the drive signal output from the drive circuit 26 are input to the I arm and the Q arm of the second modulation unit 30, respectively.
The second branched light is phase-modulated by the second modulation unit 30 to become second modulated light, and is output from the second modulation unit 30. At this time, part of the second modulated light is input to the second internal photoelectric element 34. The second internal photoelectric element 34 converts the input optical signal of the second modulated light into an electrical signal.
The output light monitoring unit 23 monitors the light intensity of the second modulated light based on the output from the second internal photoelectric element 34 and inputs the monitoring result to the control unit 24. Specifically, the output light monitoring unit 23 detects the amplitude and phase of the demodulated pilot signal by extracting the AC component of the electrical signal output from the second internal photoelectric element 34. Thereby, the output light monitoring unit 23 monitors the light intensity of the second modulated light, similarly to the case of the first modulated light.
The control unit 24 controls the bias voltage applied to the second modulation unit 30 based on the monitoring result input from the output light monitoring unit 23. For example, when it is determined from the monitoring result of the output light monitoring unit 23 that the light intensity of the second modulated light is larger than a desired value, the light intensity attenuates the bias voltage applied to the second modulation unit 30. To control. At this time, the control unit 24 refers to the phase of the demodulated pilot signal to determine whether the bias voltage should be controlled to be increased or decreased.
The rotor 31 rotates the polarization plane of the second modulated light. Specifically, the rotator 31 rotates the polarization plane of the second modulated light so that the polarization plane of the second modulated light and the polarization plane of the first modulated light are orthogonal to each other.
The first modulated light and the second modulated light are combined by the polarization beam combiner 32 and output from the polarization beam combiner 32. Part of the combined light output from the polarization beam combiner 32 is input to the external photoelectric element 27. The external photoelectric element 27 converts the input combined light into an electric signal.
The output light monitoring unit 23 monitors the light intensity of the combined light based on the output from the external photoelectric element 27 and inputs the monitoring result to the control unit 24.
Here, the control unit 24 determines whether or not the light intensity of the combined light has a desired value based on the monitoring result of the light intensity of the combined light input from the output light monitoring unit 23.
When it is determined that the light intensity of the combined light does not have a desired value, the control unit 24 controls the bias circuit 25 to control the bias voltage input to the first modulation unit 29 and the second modulation unit 30. To do. Here, the control unit 24 controls the bias voltage so that the light intensity of the combined light becomes a desired value and the light intensity of the first modulated light and the light intensity of the second modulated light become the same value. To do. Whether or not the light intensity of the first modulated light and the light intensity of the second modulated light are the same is monitored for the light intensity of the first modulated light sequentially input from the output light monitoring unit 23. A determination is made based on the result and the monitoring result of the light intensity of the second modulated light.
Next, a more specific flow of the operation of the optical transmitter 20 will be described with reference to FIG.
First, at step 10, a target value of light intensity is set. In other words, 2X is set as the target value of the light intensity of the combined light output from the optical transmitter 20. Further, it is assumed that X (2X / 2) is set as the target value of the light intensity of the first modulated light and the second modulated light (step 10). The amplitude value of the drive signal input to each arm is constant, and the value is 2Vπ. The operating point of the bias voltage is set to a NULL point in the modulation curve as an initial value. It is assumed that light from the light source 21 is input to the modulator 22 having such settings.
The light output from the light source 21 is first branched into first branched light and second branched light by the branching unit 28.
Then, the first branched light is modulated into the first modulated light by the first modulation unit 29. Part of the first modulated light output from the first modulation unit 29 is input to the first internal photoelectric element 33 and converted into an electrical signal. The output light monitoring unit 23 extracts a pilot signal that is an AC component from the electrical signal output from the first internal photoelectric element 33. Then, the light intensity of the first modulated light is determined based on the information on the amplitude and phase of the extracted pilot signal and information on the relationship between the modulation curve and the pilot signal recorded in advance. The output light monitoring unit 23 notifies the control unit 24 of information on the light intensity of the first modulated light. The control unit 24 determines whether or not the notified light intensity of the first modulated light matches the target value X (step 11).
If it is determined that the light intensity of the first modulated light is greater than the target value X, that is, if NO in step 11, the bias voltage is controlled (step 12). The control of the bias voltage at this time is control for shifting the operating point of the bias voltage from the point at which the light intensity becomes maximum in the modulation curve, that is, from the NULL point. Whether the operating point of the bias voltage is changed in the direction in which the bias voltage increases or decreases is determined based on the phase of the demodulated pilot signal.
In this way, the light intensity of the first modulated light is adjusted to the target value X. When the light intensity of the first modulated light matches the target value X, the process proceeds to step 13.
Similarly, by controlling the bias voltage applied to the second modulation unit 30 (steps 11 and 12), the light intensity of the second modulated light is also adjusted to the target value X. That is, the control unit 24 determines whether or not the notified light intensity of the second modulated light matches the target value X (step 11). If it is determined that the light intensity of the second modulated light is greater than the target value X, that is, if NO in step 11, the bias voltage is controlled (step 12). In this way, the light intensity of the second modulated light is also adjusted to the target value X. When the light intensity of the second modulated light matches the target value X, the process proceeds to step 13.
After the polarization plane of the second modulated light is rotated by the rotor 31, the first modulated light and the second modulated light are combined by the polarization beam combining unit 32. Part of the combined light output from the polarization beam combiner 32 is input to the external photoelectric element 27 and converted into an electrical signal. The output light monitoring unit 23 monitors the light intensity of the combined light based on the electrical signal output from the external photoelectric element 27 and inputs the monitoring result to the control unit 24.
The control unit 24 determines from the input monitoring result whether the light intensity of the combined light matches the target value 2X (step 13). Here, it is assumed that the light intensity of the combined light is 2X−α, which is different from the target value 2X, and the light intensity of the combined light does not match the target value, that is, NO in step 13. Although the light intensity of the first modulated light and the second modulated light is set to X, the reason why the light intensity of the combined light does not become 2X is that the propagation loss when the modulated polarized wave propagates through the transmission path, This is because optical loss such as insertion loss of the rotor 31 and the polarization beam combiner 32 occurs.
In this case, the control unit 24 resets the target value of the first modulated light, and changes the target value of the first modulated light from X to X + (α / 2) (step 14). Similarly, the target value of the second modulated light is reset, and the target value of the second modulated light is changed from X to X + (α / 2) (step 14).
When the resetting of the light intensity target value is completed in step 14, the process returns to

steps

11 and 12. That is, the control unit 24 controls the bias voltage so that the light intensities of the first modulated light and the second modulated light become the reset target values.
Then, Steps 11 to 14 are repeated until it is determined in Step 13 that the light intensity of the combined light has reached the target value 2X. If it is determined in step 13 that the light intensity of the combined light has reached the target value 2X, the control is completed (step 15).
However, even if the control is completed (step 15), the light intensity of the first modulated light, the second modulated light, or the combined light may deviate from the target value again as time elapses. In this case, the control unit 24 resumes control of the bias voltage. After the light intensity is adjusted to the target value, the cause of the deviation from the target value again includes, for example, a change in the use environment temperature of the optical transmitter 20.
As described above, the bias voltage is controlled by the control unit 24.
Thus, in this embodiment, the light intensity of the output light can be adjusted to a desired value by controlling the bias voltage applied to the first modulation unit 29 or the second modulation unit 30. Therefore, it is not necessary to add a VOA inside and outside the optical transmitter 20, leading to cost reduction. In particular, when the optical transmitter 20 in the present embodiment is applied to an optical transmitter in a ROADM (Reconfigurable Optical Add / Drop Multiplexer) system or the like, the cost can be greatly reduced.
Furthermore, as the other effects obtained by using the optical transmitter 20 of the present embodiment, there are the following two.
First, when the optical transmitter 20 in the present embodiment is used as a coherent optical transmitter, the optical receiver characteristics in coherent communication can be stabilized.
The optical output fluctuation in a coherent optical transmitter that performs QPSK modulation, which is generally available in the current market, is a specification of ± 3 to 4 dB in consideration of output fluctuation in EOL (End of Life). This value is considerably larger than the specification of optical output fluctuation in an IM-DD (Intensity Modulation-Direct Detection) modulator. Therefore, when a coherent optical transmitter that performs QPSK modulation is used, the magnitude of the output fluctuation causes the reception characteristics of the optical receiver to deteriorate. On the other hand, when the optical transmitter according to this embodiment is used, the optical output intensity can be controlled to a desired value within the optical transmitter. Therefore, according to the optical transmitter 20 in the present embodiment, even when applied to coherent communication, it is possible to reduce the optical output fluctuation of the coherent transmitter without adding a VOA outside the optical transmitter. It becomes. Thereby, the receiving characteristic of the optical receiver can be stabilized.
Second, by using the optical transmitter 20 in the present embodiment, it is possible to suppress the difference in light intensity between the polarized waves. Hereinafter, the difference in light intensity between the polarized waves will be referred to as an inter-polarization deviation.
Generally, when polarization synthesis is performed after the light is split and modulated, a deviation between polarizations is generated due to a difference in propagation loss of each polarization. For example, the difference between the actual light intensity of the first modulated light and the light intensity of the second modulated light in the case where the light intensity of the first modulated light and the second modulated light is controlled to be maximum is the difference between the polarizations. Deviation.
When this deviation between polarizations is large, it causes deterioration in reception sensitivity when the transmitted output light is received by the optical receiver. Therefore, it is desirable that the deviation between polarizations is small. Here, in the present embodiment, the first internal photoelectric element 33 and the second internal photoelectric element 34 monitor the light intensity of each of the first modulated light and the second modulated light. Therefore, by making the target value of the light intensity of each modulated light the same value, it is possible to suppress the deviation between polarizations.
In the present embodiment, the light intensity of the first modulated light and the second modulated light is monitored by detecting the amplitude of the pilot signal. However, the present invention is not limited to this. That is, the light intensity of the output light may be monitored by extracting the DC component instead of the AC component of the electrical signal output from the first internal photoelectric element 33 and the second internal photoelectric element 34.
Furthermore, in the present embodiment, the target value of the light intensity of the first modulated light and the second modulated light is set to the same value, but the present invention is not limited to this. That is, a difference may be generated between the target values of the light intensity of the first modulated light and the second modulated light as long as the difference does not cause deterioration in reception sensitivity on the receiver side. For example, when the deviation between the light intensity of the combined light and the target value is very small in step 13, only the target value of either the first modulated light or the second modulated light is changed in step 14. It is also good.
Further, in the present embodiment, one output light monitoring unit 23 uses the first modulated light, the second modulated light, and the second modulated light based on the outputs from the first internal photoelectric element 33, the second internal photoelectric element 34, and the external photoelectric element 27. Although the light intensity of the modulated light and the combined light is monitored, the present invention is not limited to this. For example, as shown in FIG. 7, an internal output light monitoring unit 36 to which outputs from the first internal photoelectric element 33 and the second internal photoelectric element 34 are input, and an external output to which output from the external photoelectric element 27 is input. It is good also as dividing with the optical monitoring part 37. FIG.
Furthermore, in the present embodiment, the output light monitoring unit 23 records information on the relationship between the modulation curve of the modulator of each arm and the pilot signal, but the present invention is not limited to this. For example, the control unit 24 may record these information.
When controlling the first modulation unit 29 or the second modulation unit 30, it is preferable to match the light intensity of the output light from the I arm and the light intensity of the output light from the Q arm. That is, it is preferable to control the first modulation unit 29 or the second modulation unit 30 while maintaining a balance between the I component and the Q component as in the constellation map shown in FIG. 8A. If the light intensity of the output light from the I arm and the light intensity of the output light from the Q arm are significantly different, as shown in FIG. 8B, the balance between the I component and the Q component is lost, and the orthogonality of the optical signal This is because the property is lowered.
In order to align the light intensity of the output light from the I arm and the output light from the Q arm, for example, the control unit 24 refers to the modulation curve of each arm, and the first modulation unit 29 or the second modulation. The unit 30 may be controlled.
In general, it is considered undesirable for the operating point of the bias voltage to deviate from the NULL point, that is, the point where the light output intensity is maximum, because this leads to signal degradation. In the case of the NRZ (Non-Return-to-Zero) system, it is considered undesirable that the operating point of the bias voltage deviates from the QUADRATURE point, that is, the point where the optical output intensity is maximum, because this leads to signal degradation. . Therefore, in the optical transmitters and modulators described in Patent Documents 1 to 3, the magnitude of the bias voltage is controlled so that the optical output intensity becomes a maximum value. However, in the present embodiment, the control for setting the optical output intensity of the optical transmitter to a predetermined value includes the control for deliberately shifting the operating point of the bias voltage from the point at which the optical output intensity is maximized. . Hereinafter, the reason will be described based on the experimental data shown in FIGS.
FIG. 9 shows the relationship between the bias voltage applied to the I arm and Q arm of the LN modulator used in the QPSK modulation method and the amplitude of the demodulated pilot signal. The amplitude and frequency of the pilot signal superimposed on the bias voltage were 120 mVpp and 1 kHz, respectively. Here, the position (point C) at which the amplitude of the demodulated pilot signal becomes zero in FIG. 9 indicates the case where the operating point of the bias voltage is set to the NULL point in the modulation curve.
10A to 10E show the waveform of the demodulated pilot signal and the signal waveform of the output light when the bias voltage is adjusted to a value corresponding to points A to E in FIG. The upper diagrams in FIGS. 10A to 10E show the demodulated pilot signal waveforms, and the lower diagrams in FIGS. 10A to 10E show the output light signal waveforms. The bias voltages corresponding to points A to E in FIG. 9 are -2.569 V (point A), -1.142 V (point B), -0.428 V (point C), 0.142 V (point D), respectively. And 1.427 V (point E). Further, when the operating point of each bias voltage is indicated by a deviation from the NULL point of the modulation curve, −Vπ / 2 (point A), −Vπ / 4 (point B), zero (point C), + Vπ / 4. (Point D) and + Vπ / 2 (Point E). Here, Vπ is the magnitude of the voltage necessary for changing the phase of light by π in the modulation curve. It should be noted that the point C is slightly deviated from the position where the amplitude of the pilot signal becomes zero, which is due to the accuracy of bias voltage control of the apparatus used in the experiment. That is, it is difficult to accurately match the operating point of the bias voltage to the NULL point in the modulation curve, and some errors will occur.
Here, the width of the horizontal bar connecting the inverted triangular shapes appearing in the output waveforms of the output light shown in FIGS. 10A to 10E is defined as a width d. If the width d is longer than the width d when the operating point of the bias voltage is set to a NULL point in the modulation curve (FIG. 10C), signal degradation has occurred.
10B and 10D, it can be seen that the width d in the output waveform when the operating point of the bias voltage is changed ± Vπ / 4 from FIG. 10C is almost the same as the width d in FIG. 10C. That is, it can be seen that even if the operating point of the bias voltage is changed by ± Vπ / 4 from the NULL point in the modulation curve, signal degradation hardly occurs.
On the other hand, it can be seen from FIGS. 10A and 10E that the width d in the output waveform when the operating point of the bias voltage is changed ± Vπ / 2 from FIG. 10C is considerably larger than the width d in FIG. 10C. That is, it can be seen that signal deterioration occurs when the operating point of the bias voltage is changed ± Vπ / 2 from the NULL point in the modulation curve.
From the above results, it was found that if the bias voltage is varied in the range of ± Vπ / 4 from the NULL point in the modulation curve, the signal deterioration does not occur so much. That is, even if the operating point of the bias voltage is shifted from the NULL point in the modulation curve, the light intensity can be attenuated with almost no signal deterioration if the deviation is in the range of ± Vπ / 4. I understood.
For this reason, the control of the bias voltage of the optical transmitter 20 in the present embodiment includes the control for attenuating the light intensity by shifting the operating point of the bias voltage from the point where the light intensity is maximized.
By the way, the optical output intensity of a coherent optical transmitter that performs modulation using the QPSK modulation method usually varies by about ± 3 dB. That is, when a plurality of wavelengths are multiplexed, the difference in light intensity of light having different wavelengths is usually about 6 dB at the maximum. In order to eliminate this difference in light intensity, the light intensities of the respective wavelengths may be aligned with the light intensities of the light having the smallest light intensity. In this case, for light of other wavelengths, the light intensity may be attenuated by about 6 dB at the maximum. At this time, when each light is a combined light obtained by combining two polarized waves, it is only necessary to attenuate the light intensity by a maximum of about 3 dB per one polarized wave. Further, in the I arm and the Q arm, it is only necessary to attenuate the light intensity by about 1.5 dB at the maximum. Here, the maximum light output intensity of the modulator used in the QPSK modulation method is usually 20 dB or more. Therefore, attenuation of about 1.5 dB for each arm can be sufficiently realized by changing the operating point of the bias voltage within a range of ± Vπ / 4 from the NULL point in the modulation curve. That is, when the optical transmitter 20 according to the present embodiment is applied to a coherent optical transmitter, it is possible to correct a difference in light intensity between a plurality of wavelengths with almost no signal degradation.
In addition, although the optical transmitter 20 in this embodiment performed the optical transmission by a DP-QPSK system, it is not restricted to this. For example, the present embodiment can also be applied to an optical transmitter that performs optical transmission using a QAM (Quadrature Amplitude Modulation) method or the like. Note that the QAM method is a modulation method combining phase change and amplitude change, in which multi-level ASK (Amplitude-shift keying) is quadrature modulated.
[Third Embodiment]
An optical transmitter according to the third embodiment of the present invention will be described with reference to FIG. FIG. 11 shows a configuration of the optical transmitter 40 in the present embodiment.
The optical transmitter 40 in the present embodiment is different from the optical transmitter 20 in the second embodiment in that the external photoelectric element 27 is not included. In addition, the optical transmitter 40 includes a recording unit 41 that records information about optical loss. The recording unit 41 is a recording medium such as a ROM (Read Only Memory). Since other configurations are the same as those of the optical transmitter 20, description thereof is omitted.
The recording unit 41 records information on the amount of light loss of the first modulated light output from the first modulation unit 29 and information on the amount of light loss of the second modulated light output from the second modulation unit 30. Yes. The information regarding the optical loss amount of the first modulated light is, for example, the propagation loss of the first modulated light from when the first modulated light is output from the first modulator 29 to when it is output from the polarization beam combiner 32. Or the quantum efficiency of the first internal photoelectric device. Similarly, the information regarding the optical loss amount of the second modulated light is, for example, the second modulated light from when the second modulated light is output from the second modulator 30 to when it is output from the polarization beam combiner 32. And the quantum efficiency of the second internal photoelectric device. The information regarding the optical loss amount may further include the insertion loss amount of the rotor 31 or the polarization beam combiner 32. The insertion loss amount of the rotor 31 and the polarization beam combiner 32 is the amount of light loss of the first modulated light and the second modulated light due to the insertion of the rotor 31 and the polarization beam combiner 32.
Next, the operation of the optical transmitter 40 will be described.
Until the control unit 24 controls the bias voltage applied to the first modulation unit 29 and the second modulation unit 30 based on the monitoring result of the light intensity of a part of the first modulation light and the second modulation light, Since this is the same as steps 10 to 12 in the second embodiment, description thereof is omitted. Hereinafter, a process in which the optical transmitter 40 monitors the light intensity of the combined light output from the polarization beam combiner 32 will be described.
The output light monitoring unit 23 of the optical transmitter 40 receives the output from the first internal photoelectric element 33 and the second internal photoelectric element 34 and information on the amount of light loss recorded in the recording unit 41 from the polarization beam combining unit 32. The light intensity of the output synthesized light is calculated. That is, the light intensity of the first modulated light is calculated from the output from the first internal photoelectric element 33, and the light intensity of the second modulated light is calculated from the output from the second internal photoelectric element 34. Then, the light intensity of the combined light is calculated by subtracting the amount of light loss recorded in the recording unit 41 from the sum of the light intensity of the first modulated light and the light intensity of the second modulated light. For example, it is assumed that the light intensity of the first modulated light is 10 dB and the light intensity of the second modulated light is 10 dB. Then, it is assumed that the information regarding the light loss amount recorded in the recording unit 41 is the light loss amount 0.5 dB of the first modulated light and the light loss amount 0.5 dB of the second modulated light. In this case, the output light monitoring unit 23 calculates the light intensity of the combined light as 10 + 10− (0.5 + 0.5) = 19 dB.
Usually, the amount of light loss that occurs between the time when the first modulated light and the second modulated light are output from the first modulation unit 29 or the second modulation unit 30 until they are output from the polarization beam combining unit 32 is It is constant regardless of the light intensity of the light and the second modulated light. Therefore, by recording this amount of light loss in the recording unit 41, the light intensity of the synthesized light can be calculated without providing the external photoelectric element 27 as in the second embodiment.
When the output light monitoring unit 23 calculates the light intensity of the combined light, the output light monitoring unit 23 sends the calculation result to the control unit 24. Then, the control unit 24 controls the first modulation unit 29 and the second modulation unit 30 based on the monitoring result of the light intensity of the combined light transmitted from the output light monitoring unit 23. Since the operation after the monitoring result of the light intensity of the combined light is sent is the same as

steps

14 and 15 in the second embodiment, the description thereof is omitted.
As described above, also in this embodiment, as in the second embodiment, it is not necessary to add a VOA inside and outside the optical transmitter, and the cost can be reduced. Further, when the optical transmitter 40 is used as the coherent optical transmitter, the optical receiver characteristics in the coherent communication can be stabilized. Further, it is possible to suppress the polarization deviation.
Further, unlike the optical transmitter 20, the optical transmitter 40 can monitor the light intensity of the combined light without adding the external photoelectric element 27. Therefore, the optical transmitter 40 can further reduce cost and downsize the transmitter as compared with the optical transmitter 20.
Furthermore, the optical transmitter 40 in the present embodiment records the amount of light loss of the first modulated light and the amount of light loss of the second modulated light. Therefore, the control unit 24 can set the target value of the light intensity of the first modulated light and the second modulated light according to the difference in the amount of light loss between the first modulated light and the second modulated light. For example, assume that the amount of light loss generated in the first modulated light is 1 dB and the amount of light loss generated in the second modulated light is 1.5 dB. In this case, the control unit 24 considers that the amount of light loss between the two is 0.5 dB different, and the target value of the light intensity of the first modulated light and the target value of the light intensity of the second modulated light are also different by 0.5 dB. Set with. That is, the target value of the light intensity of the first modulated light is made smaller by 0.5 dB than the target value of the light intensity of the second modulated light. Thereby, the optical transmitter 40 in the present embodiment can further reduce the deviation between polarizations of the first modulated light and the second modulated light included in the combined light.
Although the recording unit 41 is provided in the present embodiment, the present invention is not limited to this. For example, the output light monitoring unit 23 may include a recording unit and record information regarding optical loss. Alternatively, the control unit 24 may incorporate a recording unit and record information regarding optical loss.
Also in this embodiment, as in the second embodiment, it is desirable to make the light intensity of the output light of the I arm and Q arm uniform.
[Fourth Embodiment]
An optical transmitter according to the fourth embodiment of the present invention will be described. The optical transmitter 50 in the present embodiment is similar in configuration to the optical transmitter 20 in the second embodiment, but operates differently.
That is, in the optical transmitter 20 of the second embodiment, the control unit 24 controls the bias voltage applied to the first modulation unit 29 and the second modulation unit 30, so that the first modulation light and the second modulation light are applied. It was decided to control the light intensity of light and synthetic light. On the other hand, in the optical transmitter 50 of the present embodiment, the control unit 24 controls the amplitudes of the drive signals input to the first modulation unit 29 and the second modulation unit 30, so that the first modulated light and the second modulation light are transmitted. Control the light intensity of the light and the combined light.
The fact that the light intensity of the output light can be controlled by controlling the amplitude of the drive signal will be described with reference to FIGS.
FIG. 5 shows a graph of modulation curves in the I arm and Q arm of the modulator 22 as described above. FIG. 5 also shows the amplitude of the drive signal, which is 2Vπ here. On the other hand, FIG. 12 shows a case where the amplitude of the drive signal is set to a value α smaller than 2Vπ. It is assumed that the operating point of the bias voltage is set to the NULL point of the modulation curve as in the case shown in FIG.
From FIG. 12, it can be seen that by shifting the amplitude of the drive signal from 2Vπ by α, the light intensity of the output light decreases and the amplitude of the demodulated pilot signal increases. That is, it can be seen that the light intensity of the output light can be controlled by controlling the amplitude of the drive signal.
Using this principle, the optical transmitter 50 controls the amplitudes of the drive signals input to the first modulation unit 29 and the second modulation unit 30, so that the first modulated light, the second modulated light, and the combined light are mixed. Control light intensity.
Next, the operation of the optical transmitter 50 will be described in detail with reference to FIG. Note that steps 10, 11, and 13 to 15 in FIG. 13 are the same as the operation of the optical transmitter 20, and thus the description thereof is omitted. Hereinafter, step 16 which is an operation different from that of the optical transmitter 20 will be described.
If it is determined in step 11 that the light intensity of the first modulated light does not match the target value, the control unit 24 controls the amplitude of the drive signal input to the first modulation unit 29 (step 16). For example, when it is determined that the light intensity of the first modulated light is greater than the target value, control is performed to shift the amplitude of the drive signal from 2Vπ. Here, the control unit 24 records the relationship between the amplitude of the drive signal and the amplitude and phase of the pilot signal as shown in FIGS. Thus, the control unit 24 determines the amplitude of the drive signal for making the light intensity of the first modulated light coincide with the target value, and notifies the drive circuit 26 of the amplitude. Then, the drive circuit 26 inputs a drive signal having the amplitude notified from the control unit 24 to the first modulation unit 29. The amplitude of the drive signal output from the drive circuit 26 can be monitored by a peak detection function that the drive circuit 26 has.
In this way, the control unit 24 controls the first modulation unit 29. Similarly, the control unit 24 controls the second modulation unit 30 so that the light intensity of the second modulated light matches the target value.
As described above, the optical transmitter 50 in the present embodiment controls the amplitude of the drive signal input to the first modulation unit 29 and the second modulation unit 30, so that the light intensity of the output light from the optical transmitter 50 is controlled. Can be set to a desired value.
Therefore, also in this embodiment, the same effect as the second embodiment can be obtained. That is, it is not necessary to add a VOA inside and outside the optical transmitter 20, and the cost can be reduced. Moreover, when the optical transmitter 50 is used as a coherent optical transmitter, the optical receiver characteristics in coherent communication can be stabilized. Further, it is possible to suppress the polarization deviation.
In general, it is not preferable that the amplitude of the drive signal is 2Vπ, and in the NRZ system, Vπ, that is, deviating from the value at which the optical output intensity becomes maximum, is caused by signal deterioration. For this reason, in the optical transmitters and modulators described in Patent Documents 1 to 3, the amplitude of the drive signal is controlled to be a value that maximizes the optical output intensity. However, in the present embodiment, the control for setting the optical output intensity of the optical transmitter to a predetermined value includes the control for deliberately shifting the amplitude of the drive signal from the value at which the optical output intensity is maximized.
This is because if the range in which the amplitude of the drive signal is varied is within a predetermined range, the light intensity can be attenuated with little signal degradation.
The range in which the amplitude of the drive signal can be varied with almost no signal deterioration is within a range of ± Vπ / 2 from the amplitude value of the drive signal at which the optical output intensity is maximum.
[Fifth Embodiment]
By the way, when the coherent optical transmitter is applied to a WDM system, when the optical output intensity of the coherent optical transmitter fluctuates, the level deviation between channels of the WDM signal, that is, the tilt increases. FIG. 14 shows the light intensities of a plurality of channels having different wavelengths. In FIG. 14, the second channel from the left has a higher light intensity than the other channels, and tilt occurs.
Furthermore, in a general long-distance WDM system, a plurality of EDFAs (Erbium Doped Fiber Amplifiers) are used to perform optical amplification in multiple stages. Therefore, the increase in tilt has a great influence on the system. In particular, it affects the transmission distance, transmission bandwidth, etc. in the WDM system. This is because in order to maintain transmission quality, the guarantee of optical signal to noise ratio (OSNR) is a key point, but the OSNR for each channel greatly changes due to the increase in tilt. It is.
For example, ASE (Amplified Spontaneous Emission) light is used as the light source of the WDM signal.
Therefore, in the fifth embodiment of the present invention, a wavelength multiplexing transmission apparatus capable of suppressing an increase in tilt will be described.
FIG. 15 shows the configuration of the wavelength division multiplex transmission apparatus 60 in the present embodiment. The wavelength division multiplex transmission apparatus 60 includes a plurality of optical transmitters 10 according to the first embodiment. The plurality of optical transmitters 10 included in the wavelength division multiplexing transmission device 60 are respectively optical transmitters 10. ₁ ~ 10 _M And Also, the optical transmitter 10 ₁ ~ 10 _M Each output light of different wavelengths. The wavelength multiplexing transmission device 60 further includes the optical transmitter 10. ₁ ~ 10 _M A wavelength multiplexing unit 61 that multiplexes the wavelengths output from the respective units is provided.
Next, the operation of the wavelength division multiplexing transmission device 60 will be described.
First, the optical transmitter 10 ₁ ~ 10 _M A target value of the light intensity of the combined light is set in each control unit. At this time, the target value to be set is the optical transmitter 10. ₁ ~ 10 _M All values are common.
Next, the optical transmitter 10 ₁ ~ 10 _M The control units respectively control the first modulation unit and the second modulation unit based on the monitoring result of the output light monitoring unit. Optical transmitter 10 at this time ₁ ~ 10 _M These operations are the same as steps 1 to 5 described in the first embodiment. Optical transmitter 10 ₁ ~ 10 _M When all the light intensities of the output lights coincide with the target value, the control is completed.
And the optical transmitter 10 ₁ ~ 10 _M The light output from each is wavelength multiplexed by the wavelength multiplexing unit 61 and output from the wavelength multiplexing transmission device 60.
As described above, in the present embodiment, the plurality of optical transmitters 10 included in the wavelength division multiplexing transmission device 60. ₁ ~ 10 _M The light intensity of the output light output from can be made to a common target value.
Therefore, according to the wavelength division multiplex transmission apparatus 60 in the present embodiment, it is possible to suppress an increase in tilt. Thereby, it becomes possible to suppress deterioration of communication characteristics.
Note that the optical transmitter 10 in the present embodiment. ₁ ~ 10 _M The target value of the output light set for each may be an arbitrary value, but is not limited thereto. For example, it may be set as follows.
First, the optical transmitter 10 ₁ ~ 10 _M Each is operated to maximize the light output intensity. That is, the optical transmitter 10 ₁ ~ 10 _M When performing QPSK modulation, the operating point of the bias voltage applied to each arm is set to the NULL point of the modulation curve. Further, the amplitude of the drive signal input to each arm is set to 2Vπ.
And the optical transmitter 10 ₁ ~ 10 _M From the monitoring result of the output light monitoring unit that each has, the optical transmitter 10 ₁ ~ 10 _M The light intensity of each output light is compared. Then, the lowest output light intensity is obtained from the optical transmitter 10. ₁ ~ 10 _M Is set as the target value of the output light intensity. That is, the optical transmitters other than the optical transmitter with the lowest light intensity of the output light perform control to attenuate the light intensity of its own output light. In addition, when setting a target value by such a method, it is necessary to provide the comparison part 62 as shown in FIG. The comparison unit 62 includes an optical transmitter 10. ₁ ~ 10 _M The monitoring results from the respective output light monitoring units 23 are input. The comparison unit 62 compares the input monitoring results and determines the target value of the output light intensity. Then, the comparison unit 62 converts the determined target value into the optical transmitter 10. ₁ ~ 10 _M To each control unit. As described above, the target value of the output light intensity may be set.
In addition, although the wavelength division multiplexing apparatus 60 of this embodiment decided to have two or more optical transmitters 10 in 1st Embodiment, it is not restricted to this. For example, a plurality of optical transmitters 20 in the second embodiment may be provided. Alternatively, a plurality of optical transmitters 40 in the third embodiment and a plurality of optical transmitters 50 in the fourth embodiment may be provided.
In the present embodiment, the optical transmitter 10 ₁ ~ 10 _M Although each has a light source, it is not restricted to this. That is, the wavelength division multiplex transmission device 60 may include a wavelength tunable laser assembly (ITLA: Integrable Tunable Laser Assembly) that can switch wavelengths at high speed. Then, light having different wavelengths output from the wavelength tunable laser assembly is transmitted to the optical transmitter 10. ₁ ~ 10 _M It is good also as inputting into. Similarly, in this embodiment, the optical transmitter 10 ₁ ~ 10 _M Although each has a control part, it is not restricted to this. That is, the wavelength division multiplexing transmission apparatus 60 includes one control unit, and the control unit is the optical transmitter 10. ₁ ~ 10 _M It is good also as controlling each 1st modulation | alteration part and 2nd modulation | alteration part.
As mentioned above, although embodiment which concerns on this invention was described referring drawings, it cannot be overemphasized that this invention is not limited to this embodiment. Various shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.
In the first to fifth embodiments, a recording medium recording software program codes for realizing the functions of the embodiments is supplied to a communication terminal, and the computer of the communication terminal is stored in the recording medium. Needless to say, this can also be achieved by reading and executing the program code.
As the recording medium for supplying the program, for example, the above-mentioned program can be stored such as a CD-ROM (Compact Disc Read Only Memory), a DVD-R (Digital Versatile Disk Recordable), an optical disc, a magnetic disc, and a nonvolatile memory card. Anything is fine.
A part or all of the above-described embodiment can be described as in the following supplementary notes, but is not limited thereto.
(Supplementary Note 1) A modulator, an output light monitoring unit, and a control unit are provided, and the modulator branches the light input to the modulator into a first branched light and a second branched light A first modulation unit that performs phase modulation of the first branched light, a second modulation unit that performs phase modulation of the second branched light, and first modulated light output from the first modulation unit, A second rotating light output from the second modulating unit; a rotator that rotates one of the polarization planes; a polarization combining unit that combines the first modulated light and the second modulated light; The output light monitoring unit monitors the light intensity of the combined light output from the polarization beam combining unit, and the control unit performs the first modulation based on a monitoring result by the output light monitoring unit. And at least one of the second modulation unit, and the control includes at least one of the first modulated light and the second modulated light. The light intensity of the square, characterized in that includes a light intensity control to be smaller than the maximum value of the light intensity in the modulation curve, the optical transmitter.
(Supplementary note 2) The optical transmitter according to supplementary note 1, wherein the output light monitoring unit further monitors the light intensity of the first modulated light and the light intensity of the second modulated light.
(Additional remark 3) The 1st photoelectric conversion element into which the output light from said 1st modulation part is branched and input, and the 2nd photoelectric conversion element into which the output light from said 2nd modulation part is branched and input And a third photoelectric conversion element to which the output light from the polarization beam combiner is branched and input, and the output light monitoring unit outputs from the first to third photoelectric conversion elements The optical transmitter according to appendix 2, wherein the optical intensity of each of the first modulated light, the second modulated light, and the combined light is monitored based on
(Additional remark 4) The 1st photoelectric conversion element into which the output light from said 1st modulation part is branched and input, and the 2nd photoelectric conversion element into which the output light from said 2nd modulation part is branched and input And a recording unit that records information on the amount of optical loss of each of the first modulated light and the second modulated light, and the output light monitoring unit includes the first and second photoelectric conversion elements. The light intensity of each of the first modulated light, the second modulated light, and the combined light is monitored based on the output of the light and the information on the amount of light loss recorded in the recording unit The optical transmitter according to appendix 2, wherein:
(Supplementary Note 5) Information on the amount of optical loss includes information on the quantum efficiency of the first photoelectric conversion element, the quantum efficiency of the second photoelectric conversion element, and the insertion loss of the polarization beam combiner. The optical transmitter according to appendix 4, wherein at least the optical transmitter is included.
(Appendix 6) A drive unit that inputs a drive signal to the first modulation unit and the second modulation unit;
A bias circuit that applies a bias voltage to the first modulation unit and the second modulation unit, and the control unit controls the magnitude of the bias voltage output by the bias circuit, thereby The optical transmitter according to any one of appendices 1 to 5, wherein the optical intensity is controlled.
(Supplementary Note 7) The control unit sets the operating point of the bias voltage to ± Vπ / 4 (Vπ: π change in the phase of light in the modulation curve) from the operating point of the bias voltage for maximizing the light intensity in the modulation curve. The optical transmitter according to appendix 6, wherein the optical transmitter is controlled within a range of a magnitude of a voltage that can be generated).
(Supplementary Note 8) A drive unit that inputs a drive signal to the first modulation unit and the second modulation unit, a bias circuit that applies a bias voltage to the first modulation unit and the second modulation unit, The control unit performs the light intensity control by controlling the amplitude of the drive signal output from the drive unit, according to any one of appendices 1 to 5, Optical transmitter.
(Supplementary Note 9) The control unit may change the amplitude of the drive signal from the amplitude for maximizing the light intensity in the modulation curve by ± Vπ / 2 (Vπ: a voltage that can change the phase of the light in the modulation curve by π). 9. The optical transmitter according to appendix 8, wherein the optical transmitter is controlled within a range of
(Supplementary Note 10) A pilot signal having a predetermined frequency is superimposed on the bias voltage, and the output light monitoring unit outputs the phase of the pilot signal output from the first modulation unit and the second modulation unit. The optical transmitter according to any one of appendices 6 to 9, further monitoring the phase of the pilot signal to be transmitted.
(Supplementary Note 11) The output light monitoring unit monitors the light intensity of the first modulated light by detecting the amplitude of the pilot signal output from the first modulation unit, and from the second modulation unit. The optical transmitter according to appendix 10, wherein the optical intensity of the second modulated light is monitored by detecting the amplitude of the pilot signal to be output.
(Supplementary Note 12) A plurality of optical transmitters and a wavelength multiplexing unit that multiplexes wavelengths output from each of the plurality of optical transmitters, wherein each of the plurality of optical transmitters is any one of Supplementary Notes 1 to 11. A wavelength division multiplexing transmission apparatus, characterized by being an optical transmitter according to claim 1.
(Additional remark 13) The said output light monitoring result of each of these optical transmitters is input, and based on the said output light monitoring result, the target value of the optical intensity of the said synthesized light of these optical transmitters is determined. The wavelength division multiplex transmission apparatus according to appendix 12, further comprising a comparison unit.
(Supplementary Note 14) A branching process for splitting light into a first branched light and a second branched light, a first modulation process for performing phase modulation of the first branched light, and a phase modulation of the second branched light A second modulation step, a first step of modulating the first modulated light modulated by the first modulation step, and a second step of rotating the polarization plane of the second modulated light modulated by the second modulation step; Based on a polarization combining step of combining the first modulated light and the second modulated light, a monitoring step of monitoring the light intensity of the combined light combined by the polarization combining step, and a monitoring result by the monitoring step And a control step for controlling at least one of the modulator for performing the first modulation step and the modulator for performing the second modulation step. The control step includes the first modulated light and the second modulation step. At least one light intensity of the modulated light is changed to the light intensity in the modulation curve. Characterized to include a light intensity control step smaller than the maximum value, the light transmission method.
(Supplementary note 15) The optical transmission method according to supplementary note 14, wherein in the monitoring step, the light intensity of the first modulated light and the light intensity of the second modulated light are further monitored.
(Supplementary Note 16) A first photoelectric conversion step for performing photoelectric conversion of a part of the first modulated light, a second photoelectric conversion step for performing photoelectric conversion of a portion of the second modulated light, and the synthesized light A third photoelectric conversion step for performing a part of the photoelectric conversion, and in the monitoring step, the first modulated light based on the electrical signal converted by the first to third photoelectric conversion steps And the second modulated light and the combined light are monitored. The optical transmission method according to appendix 15, wherein:
(Supplementary Note 17) A first photoelectric conversion step for performing photoelectric conversion of a part of the first modulated light, a second photoelectric conversion step for performing photoelectric conversion of a portion of the second modulated light, and the first modulation A recording step of recording information on the amount of light loss of each of the light and the second modulated light, and in the monitoring step, the electrical signal converted by the first and second photoelectric conversion steps; Monitoring the light intensity of each of the first modulated light, the second modulated light, and the combined light based on the information on the amount of light loss recorded by the recording step. The optical transmission method according to appendix 15.
(Supplementary note 18) The information on the amount of light loss includes the amount of light loss generated in the first photoelectric conversion step, the amount of light loss generated in the second photoelectric conversion step, and the amount of light loss generated in the polarization combining step. 18. The optical transmission method according to appendix 17, wherein at least information on the amount of optical loss to be included is included.
(Supplementary Note 19) In the control step, by controlling the magnitude of the bias voltage applied to the first modulation unit that performs the first modulation step and the second modulation unit that performs the second modulation step, The optical transmission method according to any one of appendices 14 to 18, wherein the light intensity control is performed.
(Supplementary Note 20) In the control step, the operating point of the bias voltage is set to ± Vπ / 4 (Vπ: π of the phase of light in the modulation curve) from the operating point of the bias voltage for maximizing the light intensity in the modulation curve. 20. The optical transmission method according to appendix 19, wherein the control is performed within a range of the magnitude of the voltage that can be changed.
(Supplementary note 21) In the control step, by controlling the amplitude of the drive signal input to the first modulation unit that performs the first modulation step and the second modulation unit that performs the second modulation step, The optical transmission method according to any one of appendices 14 to 18, wherein the optical intensity control is performed.
(Supplementary Note 22) In the control step, the amplitude of the drive signal can be changed by ± Vπ / 2 (Vπ: π of the phase of light in the modulation curve) from the amplitude at which the light intensity is maximized in the modulation curve. The optical transmission method according to appendix 21, wherein control is performed within a range of voltage magnitude).
(Supplementary Note 23) A pilot signal of a predetermined frequency is superimposed on a bias voltage applied to the first modulation unit that performs the first modulation step and the second modulation unit that performs the second modulation step, and the monitoring In the step, the phase of the pilot signal output from the first modulation unit and the phase of the pilot signal output from the second modulation unit are further monitored. 22. The optical transmission method according to any one of 22.
(Supplementary Note 24) In the monitoring step, the light intensity of the first modulated light is monitored by detecting the amplitude of the pilot signal output from the first modulation unit, and is output from the second modulation unit. 24. The optical transmission method according to appendix 23, wherein the optical intensity of the second modulated light is monitored by detecting the amplitude of the pilot signal.
(Supplementary Note 25) A wavelength multiplexing step of multiplexing light having different wavelengths is provided, and each of the light having different wavelengths is light transmitted by the optical transmission method according to any one of Supplementary Notes 14 to 24. A wavelength division multiplexing transmission method characterized in that there is.
(Supplementary Note 26) First modulated light generated by phase-modulating the first branched light, second modulated light generated by phase-modulating the second branched light, which has a polarization plane different from that of the first modulated light, and A monitoring step of monitoring the light intensity of the combined light, and a control step of controlling at least one of the phase modulation of the first branched light and the phase modulation of the second branched light based on the monitoring result of the monitoring step And the control step includes a light intensity control step of reducing the light intensity of at least one of the first modulated light and the second modulated light to be smaller than the maximum value of the light intensity in the modulation curve. A program characterized by being included.
(Supplementary note 27) A computer-readable information storage medium that records the program according to supplementary note 26.
While the present invention has been described with reference to the preferred embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.
This application claims the priority on the basis of Japanese application Japanese Patent Application No. 2011-67700 for which it applied on March 25, 2011, and takes in those the indications of all here.

10, 10 ₁ to 10 _M , 20, 40

Optical transmitter

11, 22

Modulator

12, 23 Output

light monitoring unit

13, 24

Control unit

14, 28

Branch unit

15, 29

First modulation unit

16, 30

Second modulation unit

17, 31

Rotor

18, 32 Polarization combining unit 21 Light source 25 Bias circuit 26 Drive circuit 27 External photoelectric element 33 First internal photoelectric element 34 Second internal photoelectric element 35 ₁ to 35 ₄ Phase shift unit 36 Internal output light monitoring unit 37 External Output Light Monitoring Unit 41 Recording Unit 60 Wavelength Multiplexing Transmission Device 61 Wavelength Multiplexing Unit 62 Comparison Unit

Claims

A modulator, an output light monitoring unit, and a control unit;
The modulator is
A light branching unit that splits the light input to the modulator into a first branched light and a second branched light;
A first modulator that performs phase modulation of the first branched light;
A second modulation unit for performing phase modulation of the second branched light;
A rotor that rotates the polarization plane of any one of the first modulated light output from the first modulator and the second modulated light output from the second modulator;
A polarization beam combiner that combines the first modulated light and the second modulated light;
The output light monitoring unit monitors the light intensity of the combined light output from the polarization beam combining unit,
The control unit performs control of at least one of the first modulation unit and the second modulation unit based on a monitoring result by the output light monitoring unit,
The optical transmission is characterized in that the control includes light intensity control for making the light intensity of at least one of the first modulated light and the second modulated light smaller than the maximum value of the light intensity in the modulation curve. Machine.
The optical transmitter according to claim 1, wherein the output light monitoring unit further monitors the light intensity of the first modulated light and the light intensity of the second modulated light.
A first photoelectric conversion element into which the output light from the first modulation unit is branched and input;
A second photoelectric conversion element into which the output light from the second modulation section is branched and input;
A third photoelectric conversion element to which the output light from the polarization beam combiner is branched and input; and
The output light monitoring unit monitors the light intensity of each of the first modulated light, the second modulated light, and the combined light based on outputs from the first to third photoelectric conversion elements. The optical transmitter according to claim 2, wherein:
A first photoelectric conversion element into which the output light from the first modulation unit is branched and input;
A second photoelectric conversion element into which the output light from the second modulation section is branched and input;
A recording unit that records information on the amount of optical loss of each of the first modulated light and the second modulated light; and
The output light monitoring unit, based on outputs from the first and second photoelectric conversion elements and information on the amount of light loss recorded in the recording unit, the first modulated light, and the first The optical transmitter according to claim 2, wherein the optical intensity of each of the two modulated light and the combined light is monitored.
A drive unit that inputs a drive signal to the first modulation unit and the second modulation unit;
A bias circuit that applies a bias voltage to the first modulation unit and the second modulation unit, and
5. The optical transmitter according to claim 1, wherein the control unit performs the light intensity control by controlling a magnitude of a bias voltage output from the bias circuit. 6. .
The control unit can change the operating point of the bias voltage by ± Vπ / 4 (Vπ: the phase of light in the modulation curve by π from the operating point of the bias voltage for maximizing the light intensity in the modulation curve. The optical transmitter according to claim 5, wherein the optical transmitter is controlled within a range of voltage magnitude).
A drive unit that inputs a drive signal to the first modulation unit and the second modulation unit;
A bias circuit that applies a bias voltage to the first modulation unit and the second modulation unit, and
5. The optical transmitter according to claim 1, wherein the control unit performs the light intensity control by controlling an amplitude of the drive signal output from the drive unit. 6. .
Multiple optical transmitters;
A wavelength multiplexing unit that multiplexes wavelengths output from each of the plurality of optical transmitters,
The wavelength division multiplexing transmission apparatus according to claim 1, wherein each of the plurality of optical transmitters is the optical transmitter according to claim 1.
A branching step of branching light into a first branched light and a second branched light;
A first modulation step for performing phase modulation of the first branched light;
A second modulation step for performing phase modulation of the second branched light;
A rotation step of rotating any one of the polarization planes of the first modulated light modulated by the first modulation step and the second modulated light modulated by the second modulation step;
A polarization combining step of combining the first modulated light and the second modulated light;
A monitoring step of monitoring the light intensity of the combined light combined by the polarization combining step;
A control step of controlling at least one of a modulator that performs the first modulation step and a modulator that performs the second modulation step based on a monitoring result by the monitoring step;
The control step includes a light intensity control step of making the light intensity of at least one of the first modulated light and the second modulated light smaller than the maximum value of light intensity in a modulation curve, Optical transmission method.
Combined light of first modulated light generated by phase modulation of the first branched light and second modulated light generated by phase modulation of the second branched light, which has a polarization plane different from that of the first modulated light. A monitoring process for monitoring the light intensity of
Based on the monitoring result of the monitoring step, causing the computer to execute a control step of controlling at least one of the phase modulation of the first branch light and the phase modulation of the second branch light,
The control step includes a light intensity control step of making the light intensity of at least one of the first modulated light and the second modulated light smaller than the maximum value of light intensity in a modulation curve, program.